AQA A-Level Biology: Topic 7 Genetics, populations, evolution and ecosystems

Genotype vs Phenotype

Genotype is the genetic constitution of an organism; phenotype is the expression of this genetic constitution and its interaction with the environment.

Alleles and Diploid Organisms

Alleles are variations of a particular gene at the same locus arising by mutation; diploid organisms have 2 alleles of each gene (one on each homologous chromosome), though many alleles may exist in a population.

Dominant, Recessive, and Codominant Alleles

Dominant allele is always expressed in the phenotype; recessive allele is only expressed when two copies are present (homozygous recessive); codominant alleles are both expressed and contribute to the phenotype when inherited together.

Homozygous vs Heterozygous

Homozygous means the two alleles at a specific locus on homologous chromosomes are the same; heterozygous means the two alleles at a specific locus are different.

Monohybrid vs Dihybrid Cross

Monohybrid cross shows the inheritance of one phenotypic characteristic coded for by a single gene; dihybrid cross shows the inheritance of two phenotypic characteristics coded for by two different genes.

Pedigree Evidence for a Dominant Allele

Two parents WITH the named phenotype have a child WITHOUT the named phenotype, proving both parents must be heterozygous carriers of the recessive allele.

Pedigree Evidence for a Recessive Allele

Two parents WITHOUT the named phenotype have a child WITH the named phenotype, proving both parents must be heterozygous carriers of the recessive allele.

Sex-Linked Gene and Male Expression

A sex-linked gene has its locus on a sex chromosome (normally the X chromosome); males (XY) are more likely to express recessive X-linked alleles because they only have one X chromosome (inherited from their mother), whereas females (XX) need two recessive alleles.

Pedigree Evidence for X-Linked Recessive Inheritance

A mother without the phenotype has a son with the phenotype (she is a carrier), mainly males show the phenotype, and an affected mother always passes the allele to her sons while an affected father passes it to all his daughters.

Pedigree Evidence Against X-Linked Inheritance

An affected father has a daughter without the dominant phenotype, or an affected mother has a son without the recessive phenotype.

Autosomal Linkage

Two genes are located on the same autosome (non-sex chromosome), so their alleles are inherited together during independent segregation unless separated by crossing over (less likely if genes are close together).

Epistasis

Interaction of the products of non-linked genes where one gene masks or suppresses the expression of another gene.

When to Use a Chi-Squared (X^2) Test

Used to determine if observed categorical frequencies (such as offspring phenotypes) are significantly different from expected frequencies.

Why Observed Genetic Ratios Differ from Expected Ratios

Fertilisation of gametes is random, autosomal linkage, epistasis, or sex-linkage may occur, sample size may be too small to be representative, or some genotypes may be lethal.

Calculating and Interpreting Chi-Squared Values

Degrees of freedom = number of categories minus 1; compare X^2 to the critical value at p = 0.05; if X^2 is greater than the critical value, reject the null hypothesis because the difference is significant (less than 5% probability it is due to chance).

Population, Gene Pool, and Allele Frequency

A population is a group of organisms of the same species in one area at one time that can interbreed; a gene pool is all the alleles of all the genes in a population at one time; allele frequency is the proportion of an allele in a gene pool.

Hardy-Weinberg Principle and Conditions

States that allele frequencies in a population remain constant from generation to generation provided the population is large, there is no immigration or emigration, no mutations, no natural selection, and mating is random.

Hardy-Weinberg Equations

p + q = 1 and p^2 + 2pq + q^2 = 1, where p is dominant allele frequency, q is recessive allele frequency, p^2 is homozygous dominant frequency, 2pq is heterozygous frequency, and q^2 is homozygous recessive frequency.

Sources of Phenotypic Variation in a Population

Genetic factors (mutations as primary source, crossing over and independent segregation in meiosis, random fertilisation of gametes) combined with environmental factors (such as food availability or light intensity).

Evolution and Natural Selection Principles

Evolution is the change in allele frequency over many generations in a population; random mutations create new alleles, giving a selective advantage under a selection pressure so possessors survive, reproduce, and pass the advantageous allele on, increasing its frequency in the gene pool.

Stabilising vs Directional Selection

Stabilising selection favours the average/modal trait in a stable environment (e.g. human birth weight), reducing standard deviation; directional selection favours one extreme trait in a changing environment (e.g. antibiotic resistance), shifting the mean.

Disruptive Selection

Organisms with alleles for either extreme variation of a trait have a selective advantage over the average trait, increasing the frequency of both extremes and potentially leading to speciation.

Allopatric Speciation

A population is split by geographical isolation, causing reproductive isolation and preventing gene flow; different random mutations and selection pressures act on each population, changing allele frequencies until they can no longer interbreed to produce fertile offspring.

Sympatric Speciation

Reproductive isolation occurs within the same habitat without geographical isolation (e.g. due to mutations causing gamete incompatibility, different breeding seasons, or different courtship behaviour), preventing gene flow so different selection pressures change allele frequencies until new species form.

Genetic Drift, Bottleneck Effect, and Founder Effect

Genetic drift is a change in allele frequency due to chance rather than selection, having the strongest effect in small populations (such as after a sudden population reduction/bottleneck or when a small new colony forms/founder effect), reducing genetic diversity.

Community, Ecosystem, and Niche

A community is all populations of different species living in the same habitat at the same time; an ecosystem is a community plus its non-living (abiotic) environment; a niche is the specific role of a species within its habitat, governed by biotic and abiotic adaptations (two species cannot occupy the exact same niche).

Carrying Capacity and Abiotic Factors

Carrying capacity is the maximum stable population size an ecosystem can support; favourable abiotic factors (e.g. higher light intensity increasing photosynthesis) increase plant carrying capacity, providing more habitats and food sources to raise animal carrying capacity.

Interspecific vs Intraspecific Competition

Interspecific competition occurs between different species for shared resources, reducing population sizes or leading to competitive exclusion; intraspecific competition occurs between individuals of the same species as population density rises, regulating population size around carrying capacity.

Predator-Prey Population Cycles

Prey increase provides more food so predators increase; more predators eat more prey so prey decrease; less food causes predators to decrease; fewer predators allow prey to recover, with predator peaks lagging behind prey peaks.

Estimating Non-Motile Population Size (Random Quadrats)

Divide area into a grid with two tape measures, generate random coordinates using a random number generator to avoid bias, place a quadrat and count individuals, repeat at least 10 times to find a mean per quadrat, and calculate total population = (total habitat area / quadrat area) x mean per quadrat.

Estimating Motile Population Size (Mark-Release-Recapture)

Capture a sample, mark harmlessly, release and allow time to disperse evenly, capture a second sample, and calculate Population = (number in sample 1 x number in sample 2) / number marked in sample 2.

Assumptions of Mark-Release-Recapture

Marked individuals have sufficient time to mix evenly, marks do not rub off or affect survival/predation, there is no immigration or emigration, and there are no births or deaths (population size stays constant).

Primary Succession and Climax Community

Pioneer species colonise barren land and change abiotic conditions (dying and decomposing to form water-retaining soil/humus), making the environment less hostile so better-adapted species outcompete earlier ones; biodiversity increases until a stable climax community is reached.

Conservation Management and Plagioclimax

Conservation manages succession (e.g. by grazing or controlled burning) to prevent a climax community forming, maintaining a plagioclimax so early habitats and niches are not lost; sustainable management balances resource use with preserving biodiversity for future generations.

Required Practical 12: Random vs Systematic Sampling

Use random sampling (random coordinates on a grid) to compare two uniform areas without bias, using percentage cover if organisms are too small/numerous to count and checking a running mean for sample size; use systematic sampling (belt/line transect with quadrats at regular intervals) across an environmental gradient, analysing correlation with Spearman's rank.